"Soft" Flutter And Parameters Identification Of Nonlinear Self-excited Aerodynamic Force Of Bridge Girders

This thesis starts with the phenomenon of "soft" flutter which is unexplainable under the theory of linear aerodynamic forces. After discussions of the differences between the "soft" flutter and the vortex-induced vibration, comparisons of the flutter mechanism of bridge deck with streamline section and with blunt section, the conclusion can be obtained: the "soft" flutter is a self-equilibrium state caused by nonlinear aerodynamic damping, in which the structures absorb the energy from the air at certain range of the attack angle, and then dissipate at the other, which leads an equilibrium state.Based on the feature that the damping varies with the vibration amplitude, the concept of Van Del Pol self-excited system is employed, and a SDF nonlinear aerodynamic force formula is presented, parameters identification procedure is implemented based on the aerodynamic force formula. The phenomenon of the "soft" flutter can be satisfactorily explained by this SDF nonlinear aerodynamic force model.Finally, considering the 2-D characteristic of sectional models and the 3-D characteristic of real bridges, a formula converting the additional attack angle of sectional models to that of real bridges is derived by establishing the equilibrium equations of sectional models and real bridges respectively. The converting formula of amplitude of "soft" flutter is derived by establishing the differential equations of sectional models and real bridges respectively, using the SDF nonlinear aerodynamic force formula presented in the thesis. In the end, then the allowable amplitude of the "soft" flutter is discussed.